Field of the Invention
Embodiments described herein relate generally to a pattern inspection method and a pattern inspection apparatus. For example, Embodiments described herein relates to an inspection apparatus that inspects a pattern by acquiring a secondary electronic image of a pattern image emitted after irradiation with an electron beam.
Related Art
In recent years, with increasingly higher integration and larger capacities of large scale integrated circuits (LSI), circuit linewidths required of semiconductor devices become increasingly narrower. These semiconductor devices are manufactured by exposing and transferring a pattern onto a wafer through a reduction projection exposure apparatus called the so-called stepper using an original pattern (also called a mask or reticle. Hereinafter, generically called a mask) in which a circuit pattern is formed, to form a circuit. Thus, a pattern lithography apparatus capable of writing a fine circuit pattern and using an electron beam is used to manufacture masks to transfer such a fine circuit pattern to a wafer. Using such a pattern lithography apparatus, a pattern circuit may directly be written to a wafer. Alternatively, an attempt is being made to develop a laser beam lithography apparatus that writes a pattern using a laser beam other than the electron beam.
Improvements in yield are indispensable for the manufacture of LSI that needs a large amount of manufacturing costs. However, as representatively shown by 1 GB DRAM (random access memory), patterns constituting LSI are transitioning from the order of sub micrometer to that of nanometer. Pattern defects of a mask used to expose and transfer a superfine pattern onto a semiconductor wafer by photolithography technology can be cited as one important factor that reduces yields In recent years, with increasingly finer dimensions of LSI patterns formed on semiconductor wafers, dimensions that need to be detected as pattern defects are now extremely small. Therefore, a pattern inspection apparatus that inspects for defects of transfer masks used for the manufacture of LSI needs to be more precise.
As an inspection method, a method of conducting inspection by comparing an optical image capturing a pattern in a predetermined magnification formed on a substrate such as a lithography mask using an enlarged optical system and an optical image of design data or capturing the same pattern of a target object is known. For example, “die to die inspection” that compares optical image data capturing the same pattern in different places on the same mask and “die to database inspection” that, when writing pattern-designed CAD data to a mask as a pattern, inputs pattern writing data (design pattern data) converted into an apparatus input format for a lithography apparatus to input into an inspection apparatus, generates design image data (reference image) based thereon, and compares this data with an optical image as measurement data imaging the pattern are available as the pattern inspection method. In an inspection method in such an inspection apparatus, a target object is placed on a stage and the stage is moved for a luminous flux to scan the target object to conduct inspection. The target object is irradiated with a luminous flux by a light source and an illuminating optical system. Light transmitted or reflected by a target object is formed as an image on a sensor via an optical system. An image captured by the sensor is sent to a comparator as measurement data. After images are aligned, the comparator compares the measurement data and reference data using an appropriate algorithm and if not matched, determines that there are pattern defects.
In the above pattern inspection apparatus, a substrate is irradiated with laser light and a transmitted image or a reflected image thereof is captured to acquire an optical image. On the other hand, an inspection apparatus that acquires a pattern image by irradiating a substrate with multiple beams based on an electron beam and detecting secondary electrons corresponding to each beam emitted from the substrate is also under development (see, for example, JP-A-2011-155119). In the pattern inspection apparatus using multiple beams, a secondary electronic image obtained from the whole multiple beams is compared with a reference image. At this point, if individual distortions or gradation errors resulting from each beam constituting multiple beams arise, dummy defects determined to be defects, though not originally defects, may arise. However, it is difficult to individually correct beam characteristics or the like of each beam constituting multiple beams. In addition, even if an image obtained from the whole multiple beams is corrected, it is difficult to exclude such individual distortions or gradation errors. Countermeasures against such phenomena specific to multiple beams have not sufficiently been taken.